3. Results
Mean deep w’ variances for all 10070 profiles (computed from 1000
dbar to their deepest gridded pressure, at least 2995 dbar, typically
within ~45 dbar of the seafloor) have an average value
of 0.23 (±0.16) × 10-4 dbar-2s-2. However, the mean deep w’ variances are
not normally distributed (Figure 3a), with 3% of the values falling
below the mean minus one standard deviation and 10% of the values
falling above the mean plus one standard deviation. The median value of
0.20 × 10-4 dbar-2s-2 is somewhat less than the mean value. The mean
value corresponds to an idealized sinusoidal disturbance for w’over the entire pressure range with an amplitude of 0.007 dbar
s-1. The largest local amplitudes of w’generally do not exceed 0.02 dbar s-1. Hence, the
example shown (Figure 2) exhibits a signal with an amplitude towards the
high end of the distribution.
The vertical wavelengths of w’ at the maximum power of thew’ Morlet wavelet spectrum, hereafter referred to as the dominant
vertical wavelengths, (e.g., Figure 3b) have an average value of 890
(±290) dbar. Half of these values are 786 dbar or less, with less than
1% falling below 393 dbar, and half are 935 dbar or greater, with 6%
at the maximum possible value of 1572 dbar. This distribution should be
regarded with some caution, both because of potential biases resulting
from slow profiling through propagating waves noted in the discussion
section and because a segment 2.7 times the vertical wavelength is
required for that wavelength to be free of the cone of influence (where
zero-padding effects bias the power spectrum) even at the mid-point of
the portion of the profile analyzed. Using the 46% of the profiles that
extend to at least 5250 dbar, and hence can resolve a maximum vertical
wavelength of 1572 dbar at their mid-point, results in an average value
of 970 (±300) dbar for the dominant vertical wavelengths. This increase
in wavelength is only 9% over the average value using the entire data
set.
The geographic distributions of mean deep w’ variances (Figure 4)
and dominant vertical wavelengths (Figure 5) both exhibit physically
sensible patterns among basins and within some individual basins.
The mean deep w’ variances (Figure 4) are generally lower than
average for the profiles in the North American Basin of the western
North Atlantic Ocean, with a few higher values adjacent to the Caribbean
Islands. Offshore, the abyssal plain there is very smooth and so these
low variances might be anticipated. The dominant vertical wavelengths
there are noticeably longer than average (Figure 5). These two features
are consistent with little local generation or scattering of internal
waves at rough topographic features and propagation of internal waves
generated elsewhere from a long distance. The internal wave energy level
in regions with little local generation would be expected to be lower,
even for the longer vertical wavelength packets that survive traveling
from remote generation regions. The few bins with higher variances and
smaller wavelengths near the continental slope may result from local
interactions between currents and the bathymetry there.
In the Brazil Basin of the western South Atlantic deep w’ variance
values are largest and vertical wavelengths the smallest near the
internal wave generation sites along the rough topography of the
Mid-Atlantic Ridge. In contrast, over the smoother abyssal plain in the
west far from internal wave generation regions the mean w’ variance
values are lower and the vertical wavelengths are longer, suggesting
that there are very few locally generated internal waves, leaving only
low-mode internal waves that may have been generated elsewhere. This
pattern is consistent with previous observations of a ridge-to-basin
gradient in mixing in the Brazil Basin (Polzin et al., 1997) and with
strain and shear variances from WOCE section data in many basins (Kunze
et al., 2006). Further west, approaching the continental slope,
wavelengths become shorter while the variance remains small, suggesting
that while there is little local generation, the continental slope may
be reflecting or scattering internal waves, a pattern consistent with
modeling studies of the internal wave lifecycle (e.g., de Lavergne et
al., 2019). The shorter wavelengths near the slope could also arise at
least partly owing to the shorter profiles taken there not resolving the
energy at longer wavelengths. In the Argentine Basin vertical
wavelengths are small and the deep w’ variance is moderate to strong,
consistent with the deep-reaching eddies and currents in the region
(e.g., Fu, 2007) interacting with the internal wave field in a variety
of ways including acting as a conduit for surface-generated internal
waves (Danioux et al., 2008; Kunze, 1985; Young & BenJelloul, 1997), or
causing a substantial reduction in the internal wave length scales via
interactions with the currents vorticity or horizontal strain (Fer et
al., 2018; Kunze, 1995).
The mean deep w’ variances (Figure 4) in the South Australian and
Australian-Antarctic basins of the far eastern Indian Ocean are
generally lower than average, especially in smooth regions of the
basins, with some high values and shorter dominant vertical wavelengths
(Figure 5) closer to topographic features. Just to the east, profiles
south of the Campbell Plateau in the South Pacific have relatively high
mean deep w’ variances and a variety of dominant vertical
wavelengths. The deep-reaching Antarctic Circumpolar Current flows
eastward through this region after transiting some very rough bathymetry
to the west. Deep-reaching meanders and eddies of that current
generating lee waves when flowing over that topography (Cusack et al.,
2017; Waterman et al., 2013) may be responsible for some of those high
variances.
In the Southwest Pacific Basin, mean deep w’ variances (Figure 4)
are close to average in many locations, with higher values adjacent to
the steep bathymetry of the Tonga-Kermadec Ridge and Trench system and
around regions with rough bathymetry including the chain of seamounts
comprising the Louisville Ridge. The lower variances to the north of
that ridge in the center of the basin are in a region of smoother
bathymetry, and again the dominant vertical wavelengths (Figure 5) tend
to be longer there, consistent with propagation from remote forcing
areas of most internal waves and weaker mixing found in that region.
Moving northward in the Pacific, mean deep w’ variances (Figure 4) are
substantially higher than average within and immediately north of the
Samoan Passage and in the Penrhyn Basin east of the Manihiki Plateau.
The Samoan Passage is a constriction for northward flow of bottom water
into the rest of the Pacific, with hydraulic jumps and strong mixing
observed locally (Carter et al., 2019). The high values continuing
farther to the north of the Passage and east of the Manihiki Plateau are
perhaps partly owing to relatively rough bathymetry in those regions.
Values are also high for the few floats in the North Pacific, all of
which are close to steep topography. Moving from South to North these
floats are found in the Clarion-Clipperton Fracture Zone, around the
Hawaiian Islands, and off the continental slope just west of San Diego.